Membrane fusion is a central theme in cell biology. For example, in order to enter the host, enveloped viruses use a specialized envelope protein to catalyze fusion of the viral and host cell membranes. The recognition that a similar mechanism is used for intracellular membrane fusion emerged from the observation that cellular proteins, called SNAREs, share structural similarity with the fusion-active conformation of the viral envelope proteins. Despite the inherent ability of SNARE proteins to fuse membranes, eukaryotic cells require at least a dozen other proteins for vesicle fusion at the plasma membrane. Our goal is to determine how these proteins work together to ensure accurate vesicle targeting and fusion. One of these proteins, Sec1p, is proposed to be essential for the assembly of SNAREs into a fusion-active complex that pins membranes together, for fusion. This model gains support from the recent structure of neuronal n-Sec1 in a complex with the SNARE Syntaxin-1A. Unexpectedly, predictions of this hypothesis do not hold when tested in S. cerevisiae, a model system for secretion studies. Based on the assembly model, an absence of Sec1p function is expected to block SNARE-complex assembly. Instead, levels of SNARE complexes in the loss-of-function secretory mutant, sec1-1 are unaltered. Furthermore, unlike the neuronal counterpart, yeast Sec1p has no observable affinity for the syntaxin protein, but instead binds to assembled SNARE complexes at sites of secretion. These conflicting observations indicate that our understanding of Sec1p-dependent vesicle fusion is currently limited and, therefore, requires further study. To address this problem, we propose (I) to map on a structure of yeast Sec1p the sites required for exocytosis, using a combination of genetics and X-ray crystallography, (ii) to test hypotheses for the mechanism of fusion regulation by reconstitution of Sec1p-dependent membrane fusion and (iii) to establish whether Sec1 p homologs share a common function in vesicle fusion, by defining the SNARE-binding and membrane-fusion properties of other yeast Sec1 p homologs including Sly1 p, a yeast Sec1 p homolog with syntaxin-binding properties similar to n-Sec1.